1. Field of the Invention
The present invention is generally related to pressure monitoring systems and specifically related to systems for nonintrusively monitoring the pressure inside circuit interrupter pressure vessels.
2. Overview of the Technical Problem
High power circuit breakers or interrupters are generally an important feature in power generating and distribution facilities. Unlike circuit breakers found in the home, modern high power circuit breakers are composed of separate current sensors, an actuator assembly and 3 current interrupters (one for each phase of current). Generally, a circuit interrupter includes a switching mechanism (driven by the actuator assembly) which is sealed in a pressure vessel such as a glass epoxy bottle with a high dielectric, noninflammable gas such as sulfur hexafluoride (SF.sub.6), at 25 to 45 pounds per square inch (PSI) above the outside ambient pressure. This configuration is designed to contain and extinguish the intense arcing that is inherently present when a high power line is opened. Alternatively, the circuit interrupter switching mechanism may be housed in a vacuum container.
Unfortunately, it is impossible to insure that the seal on such a pressure vessel or bottle will remain perfectly intact. One place where a leak might develop is at the actuating shaft. This is the shaft that is used to cause the circuit interrupter to open the circuit. Also, at those places where it is necessary to run an electrical bus into the circuit interrupter, there is a leakage risk. Finally, the joints in the housing for the circuit interrupter, where one component portion meets another, are somewhat vulnerable to the development of leaks.
If too much gas leaks from the vessel, the circuit interrupter will arc violently when interrupting power to switch normal loads or respond to an excessive current fault. The tremendous heat generated by this arcing presents a grave danger to the power facility and its personnel. The same problem may occur if a vacuum bottle develops a leak which allows air into the bottle.
Because of the importance of verifying that the circuit interrupters remain in optimum working condition, they are often produced with a pressure switch or sensor already installed. Unfortunately, as these sensors directly measure the pressure inside the vessel, they require that some breach be made in the bottle to carry pressure information to the exterior of the vessel. This creates a potential leak point in the vessel.
Furthermore, most pressure switches only indicate that the interior bottle pressure is within a particular range of values. The communication from these switches is typically a simple contact closure indicating "pressure low." Although some manufacturers advise the manual manipulation of the circuit interrupter, operating shaft during periodic maintenance to further assess the level of the pressure, this yields, at best, an extremely crude measurement.
A facility operator, when advised of a "pressure low" condition in a circuit interrupter bottle, has little choice but to immediately repair the condition. This is because, although the "pressure low" indicator is generally set to signal when the pressure in the bottle is still adequate to allow switching the interrupter to its "open" state, there is no indication of how quickly the pressurized dielectric gas is leaking from the bottle. If the gas is leaking rapidly, any delay in repair could be dangerous. Should an excessive current fault occur on that circuit after the pressure has fallen to a dangerously low level, the circuit interrupter's resultant action to switch to its "open" state could result in uncontrolled arcing, producing a great deal of heat, bottle rupture and potentially a fireball.
Even if a fault does not occur when the pressure is dangerously low, the circuit interrupter will have to be switched to "open" in order to effect repairs. If the pressure has fallen sufficiently, for example, that it approximately equals the exterior ambient pressure, this switching, in itself, may cause uncontrolled arcing and the potential for fire.
The safe choice of repairing the condition immediately may, however, prove extremely expensive. Some power facility circuit interrupters control power lines that supply power to essential portions of the facility. Therefore, to service one of these circuit interrupters, the entire facility may be forced to shut down. In the case of a nuclear plant, the cost can exceed several million dollars. Even if the circuit interrupter is not on a crucial line, the repair would preferably be done on a non-emergency basis, and should be scheduled for a time when a low power demand is expected and other maintenance operations are planned.
If a facility manager can be told that the pressure in the vessel is falling very slowly, he has the option of delaying the repair until the next scheduled facility downtime. For example, this occurs every 18 months in a nuclear plant to allow the fuel to be restocked. Alternatively, he could schedule the repairs for a traditionally low power demand time period such as a Sunday night. Slow leaks are generally more common than fast leaks. Some leaks stop spontaneously as the pressure in the bottle drops below the level necessary to force out more gas.
Some circuit interrupter manufacturers advise, especially for the vacuum housed interrupters, that periodic maintenance testing be performed routinely in lieu of gauging the interior pressure of the circuit interrupter. Unfortunately, in a power facility, where the failure of a circuit interrupter could be disastrous, even the possibility that a vessel has become depressurized or has had its vacuum broken is unacceptable.
What is needed is a system for monitoring the pressure of a circuit interrupter pressure vessel without breaching the vessel to acquire the information and with sufficient precision so that repair or replacement may be scheduled to take place at a convenient time, when it is safe to do so.
3. Review of the Prior Art
A dimension sensitive transducer is a device which changes some detectable physical characteristic in response to change in a predetermined dimension of a monitored object. Strain gauges are among the most well known, widely available devices of this type. A strain gauge typically consists of a supporting substrate and a resistive element such as an elastic wire with two terminals. As the surface to which the gauge is attached changes in size along the strain gauge measurement axis, the wire geometry is changed accordingly, producing a change in electrical resistance. This change in resistance is measured to determine the change in the item's dimensions along the measurement axis.
Strain gauges are typically about an inch square with electrical resistance varying in proportion to strain about a central point that is typically in the hundreds of ohms. They can be sensitive to changes in the surface length along the measurement axis on the order of micro inches. Although there is some precedent for using electrical transducers to assess the change in volume inside a cylinder, there has been no suggestion that such a device should be used to measure the pressure inside a vessel such as a glass epoxy bottle which is used to maintain circuit interrupters in a high pressure inert gas environment or, alternatively, in a ceramic vacuum chamber.
U.S. Pat. No. 4,420,980, issued to Duneman et al., describes a method for measuring the pressure inside of a cylindrical cavity by the cooperative placement and coordination of electrical transducers to measure and compare the longitudinal and the circumferential distortion of the shape of the container. This method is intended to measure the distortions related to the interior pressure changes in diesel fuel lines and to be insensitive to other distortions which are independent of fluid pressure.
The patent granted to Atkinson et al., U.S. Pat. No. 4,706,501, teaches a method for examining the changes in pressure over time to determine when there has been a sudden, unreversed drop in pressure in a pipeline to signal the occurrence of a puncture or rupture in the line.
U.S. Pat. No. 4,117,718, granted to Hayward, teaches a method for determining whether or not the contents of a container are under the correct pressure by measuring the deflection of a flexible wall of the container at a number of closely spaced points. This method is for use with food or other containers which can be supplied with a flexible wall in monitoring.
The patent granted to Mitsukuchi et al., U.S. Pat. No. 4,723,058, describes a circuit interrupter which is mechanically disabled when a drop in the gas pressure in an enclosed volume reaches a predetermined level. The "inoperable" condition is signalled and displayed. There is no advance warning, however, as this device is either "on" or "off." Further, by making the switch inoperable, a small explosion may be prevented at the switch by risking more catastrophic events elsewhere in the system.
U.S. Pat. No. 3,263,162, granted to Lucek et al., U.S. Pat. No. 3,403,297, granted to D. W. Crouch and U.S. Pat. No. 4,403,124, granted to Perkins et al. use ion detectors to detect the presence of air molecules in what should be a vacuum. None are readily applicable to noninvasively signal pressure loss in a pressurized bottle used to isolate modern day circuit interrupters.
U.S. Pat. No. 4,000,457, granted to O'Neal III, teaches a pressure measurement device that also works with an ion detector. In this reference, however, the potential difference between anode and cathode is varied to provide for pressure measurements over a wider range of low pressure and is not directed to the problem of non-invasive pressure measurement.
Further, ionization type leak detectors that signal the presence of SF.sub.6 may signal a current leak rate, but do not report when the leak began, how long it lasted and whether the internal pressure is adequate to withstand a circuit interruption.